【发布时间】：2020-08-08 07:33:18
【问题描述】：
我正在尝试实现以下代码，以了解如何通过嵌套循环管理 OpenMP 线程，其中每个内部/外部循环在函数及其调用者中单独实现。
每个循环都是用语句实现的
#pragma omp parallel for，我假设内部循环的 pragma 被忽略。
为了看到这一点，我在每个循环中打印了线程号。
然后，我可以看到以下内容，其中内部循环中的线程 id 始终为零，与调用者对应的线程号不同。为什么会这样？
Calling 0 from 0
Calling 2 from 1
Calling 6 from 4
Calling 8 from 6
Calling 4 from 2
Calling 7 from 5
Calling 5 from 3
Calling 0 from 0 // Expecting 3
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
Calling 0 from 0
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
Calling 0 from 0
Calling 0 from 0
Calling 0 from 0
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
Calling 9 from 7
Calling 1 from 0 // Expecting 7
Calling 2 from 0
Calling 3 from 0
Calling 0 from 0
Calling 3 from 1
Calling 0 from 0 // Expecting 1
Calling 1 from 0
Calling 2 from 0
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
Calling 3 from 0
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
Calling 0 from 0
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
Calling 0 from 0
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
Calling 1 from 0
Calling 0 from 0
Calling 1 from 0
Calling 2 from 0
Calling 3 from 0
#include <vector>
#include <omp.h>
#include <iostream>
#include <cstdio>
#include <limits>
#include <cstdint>
#include <cinttypes>
using namespace std;
const size_t kM = 4;
struct Mat
{
int elem[kM];
Mat(const Mat& copy)
{
for (size_t i = 0; i<kM; ++i)
this->elem[i] = copy.elem[i];
}
Mat()
{
for (size_t i = 0; i<kM; ++i)
elem[i] = 0;
}
void do_mat(Mat& m)
{
#pragma omp parallel for
for (int i = 0; i<kM; ++i)
{
printf(" \tCalling %d from %d\n", i, omp_get_thread_num());
elem[i] += m.elem[i];
}
}
};
int main ()
{
const int kN = 10;
vector<Mat> matrices(kN);
Mat m;
#pragma omp parallel for
for (int i = 0; i < kN; i++)
{
int tid = omp_get_thread_num();
printf("Calling %d from %d\n", i, tid);
matrices[i].do_mat(m);
}
return 0;
}
【问题讨论】：
标签： c++ multithreading openmp